The glass faceplate of a CRT is comprised of a dielectric material which operates as a capacitor in storing-up an electrostatic charge as a result of the high voltages applied to the CRT. For safety reasons, this charge must be dissipated to ground. The CRT's faceplate is frequently provided with an antistatic coating on a surface thereof for bleeding the charge to ground. Antistatic coatings currently in use are generally based on three different approaches. One approach employs conductive ions such as lithium silicates in the coating. Another approach employs semiconductor materials such as comprised of tin oxides. Still another approach is based upon the use of hygroscopic materials which include ions which tend to absorb water vapor which renders the material conductive. Advantages of this last approach include low cost and ease of application to the CRT's faceplate. Problems have been encountered with this type of antistatic coating at low humidities. For example, at low humidity the bleed resistance decreases to values which allow large charges to buildup on the CRT's faceplate resulting in an unsafe condition. In addition, these hygroscopic coatings are easily scratched, particularly at low relative humidity.
Another important CRT performance characteristic involves the reflectance of its glass faceplate. Reflected light on the faceplate makes it more difficult to view a video image produced by the CRT. Various approaches have been developed to reduce the loss of image contrast due to CRT faceplate glare which is caused by random scattering of reflected light. Two basic approaches have been adopted to reduce faceplate glare, one involving the use of anti-reflective coatings and the other employs the use of antiglare coatings. Anti-reflective coatings are based upon negative reflective light interference wherein reflected light coming from the coating surface and the glass surface under the coating cancel each other for minimizing light reflection. The advantage of this type of coating is that virtually no loss of resolution occurs, but it suffers from the disadvantage of high sensitivity to fingerprints, Antiglare coatings seek to reduce random scattering of reflected light. This type of coating results in a loss of video image resolution to a certain extent, but is insensitive to fingerprints.
The prior art has combined these two approaches to reduce glare and static charge by applying a double layer of fine tin oxide particles to the CRT's faceplate. The tin oxide particles, having a diameter of about 50 nm, are suspended in a solution of ethyl silicate and ethanol. Other approaches for providing antiglare and/or antistatic coatings for a CRT are disclosed in U.S. Pat. Nos. 4,563,612; 3,689,312 and 4,785,217. A primary disadvantage of these and other prior art approaches is the relatively high cost of preparing, processing and applying the one or more coatings to the CRT's faceplate.
The present invention addresses the aforementioned limitations of the prior art by providing a two layer antiglare/antistatic coating for use on the outer surface of the faceplate of a CRT which improves viewing of the CRT's video image and provides safer CRT operation.